In internal combustion engines, EGR systems recirculate engine exhaust gas to the engine intake as a method for reducing production of oxides of nitrogen (NOx). The addition of exhaust gas to the intake air reduces the oxygen content of the intake air. This results in lower combustion temperature, and because NOx production is temperature sensitive, lower production of NOx.
Some systems use cooled EGR, that is, exhaust gas that is passed through a heat exchanger. The temperature of cooled EGR gas can get close to the dew point. Further cooling, for example by contact with cooler components, can create significant amounts of condensation. EGR condensation includes soot and other undesirable constituents which can foul EGR components. One such component subject to fouling is a venturi device used to meter the EGR gas.
Conventional solutions to the problem of EGR condensation in a venturi include a system to supply engine coolant as a heat source to maintain a temperature of the venturi walls, which adds expense and complication. Other solutions include a glass fiber insulation wrap around the venturi, which is subject to wear and tear and which requires additional assembly steps.
The invention provides a device for insulating an EGR venturi meter. According to the invention, a venturi meter includes a vacuum space surrounding the venturi body to provide thermal insulation.
A venturi meter according to the invention includes a venturi body having an inner wall defining an inlet end, a throat, and an outlet end. An outer wall surrounds the inner wall and is spaced from the inner wall to provide an insulating space between the outer wall and the inner wall. The outer wall and inner wall are joined to form a seal to prevent air from entering or leaving the insulating space.
According to a preferred embodiment, the insulating space is evacuated to provide a vacuum insulated spaced surrounding the inner wall.
According to an alternative embodiment, the insulating space contains air or another gas having low thermal conductivity.